The objectives of this research proposal are to serial section regions of known pacemaker activity in the hypostome and basal disk of Hydra in order to reconstruct neuronal architecture and synaptic connectivity patterns by electron microscopy. The neurons, axons and synapses in this simple system will be correlated with known morphology of such strctures in more highly developed systems in order to evolve a logical hypothesis for neuronal and synaptic evolution. Conventional transmission electron microscopy (CTEM) will be used to trace synaptic connections within neuronal clusters of the hypostome and nerve rings in the basal disk. Further characterization of synaptic complexes (number and types of vesicles, thickness of paramembranous coats, and width of cleft) will be done using selective stains (e.g., phosphotungstic acid) and high voltage electron microscopy (HVEM). Scanning electron microscopy (SEM) will be used to characterize surface features of sensory hairs and to correlate them with thin and semi-thick sections using CTEM and HVEM. The ultimate goal of this project is to describe the neuronal architecture of an entire multicellular organism using the techniques of electron microscopy and serial reconstruction. The technical approaches using a combination of CTEM, SEM and HVEM will have wide appplication to other fields of biomedical research at the ultrastructural level. The evolutionary hypothesis to be tested is that "primitive" nervous systems with few but multifunctional neurons have evolved into complex nervous systems with numerous and specialized neurons. We postulate that simple structural elements in synaptic complexes are common to all systems but evolutionary changes in synapses and connectivity patterns have occurred. The neurobiological analysis of a simple system can be used to explain fundamental features of more complex systems found in higher animals.